feat: extend model picker to image model (Cloudflare FLUX)

Apply the same interactive model picker UI to the image model field.
Since Cloudflare models don't come from OpenRouter's registry, the
two FLUX models are embedded as static data in the page config and
the JS popup renders cost-per-image instead of token pricing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/admin/class-settings-fields-extended.php

@@ -134,13 +134,10 @@ class PRAutoBlogger_Settings_Fields_Extended {
 			[
 				'id'          => 'prautoblogger_image_model',
 				'label'       => __( 'Image Model', 'prautoblogger' ),
-				'type'        => 'select',
+				'type'        => 'model_select',
 				'section'     => 'prautoblogger_images',
 				'default'     => PRAUTOBLOGGER_DEFAULT_IMAGE_MODEL,
-				'options'     => [
-					'flux-1-schnell' => __( 'FLUX.1 [schnell] — fast + cheap (default)', 'prautoblogger' ),
-					'flux-1-dev'     => __( 'FLUX.1 [dev] — higher quality, ~4× cost', 'prautoblogger' ),
-				],
+				'capability'  => 'cloudflare_image',
 				'description' => __( 'Schnell is the normal choice. Switch to [dev] for higher quality.', 'prautoblogger' ),
 				'badge'       => __( 'Low cost', 'prautoblogger' ),
 			],